To be fair, the Thames Tideway is storage, whereas the Elizabeth Line is transit.
That is, if you're stuck in a tunnel on an Elizabeth Line train for an hour that's a fault, everybody wants to fix that and then you'll get to Reading or, Heathrow (airport) or Woolwich or whatever.
But a turd in the Thames Tideway is supposed to be sat there, it's temporary storage, the idea is if twice as much rainfall as normal happens over a 24 hour period, we don't need twice as much sewage treatment capacity, we just need to store those turds for a while so that on average it works out OK.
Which still doesn't answer your original objection - Thames Water are shit, blame the Tories for once again taking a monopoly at point of use service which costs money and "improving" it by giving private investors a way to make money and then saying Oops when of course they take the money but leave the problem they were supposed to be solving unfixed.
In the very big picture the original problem, as for our rail infrastructure, is that England is very, very old. London has a single Combined (ie rain water and human effluent mix) sewer system because it was built many years before anybody invented separated sanitary and rain water sewers. A retro-fit upgrade, which some US cities have done, is incredibly expensive, nobody wants to pay for that even if it could be attempted. I don't anybody has even tried to estimate for London. Tens of Billions of pounds? Hundreds?